As a conventional engine there is known a free-piston engine in which a crankshaft and the like are not mechanically connected to a piston and the stroke range of the piston is not fixed. As this free-piston engine of Patent Literature 1 proposes a free-piston engine of a structure of a piston valve method in which a scavenging port, a suction port and an exhaust port, which are made on a side wall of a cylinder, are opened and closed by the movement of a piston. The free-piston engine of Patent Literature 1 is intended for incorporation in generating equipment and a pair of pistons is disposed face to face in the cylinder and a combustion space is formed between the opposed surfaces of the pistons.